This invention relates to receivers for receiving signals comprising a carrier signal modulated with a digital signal. The invention is more particularly but not exclusively concerned with such receivers intended for use in remotely-operable switching devices. The modulated carrier signals may typically be mains-borne signals, that is signals transmitted over the wires of an electrical power distribution network by suitably modulating the supply voltage of network with the modulated carried signal, or radio broadcast signals.
In the United Kingdom, it is currently being proposed that such functions as tariff-switching and load-shedding in electrical power distribution networks should be effected by means of radio-controlled switching devices connected in the power distribution circuits of individual consumers, e.g. in the consumers' electricity meters for tariff-switching and in or near the meters for load-shedding. The proposal envisages that the switching devices could be controlled from a single transmitter, e.g. the British Broadcasting Corporation's Droitwich transmitter, which would broadcast suitable control signals capable of being received over practically the whole of the United Kingdom. The control signals would be constituted by a digital signal, typically a 50 baud digital signal, carried by a long wave carrier signal, typically the 200 kHz Radio 4 signal.
However, the signal strength of the broadcast signal, as received by the radio receivers in the switching devices, will vary enormously, not only in dependence upon the location of the switching device within the United Kingdom, but also as a function of the position of the switching device within the building in which it is installed (e.g. whether it is in a basement, or near a null-point in metal-framed building). This makes the reliable detection and decoding of the digital signal extremely difficult.
It has also been proposed that such functions as tariff-switching, load-shedding and remote meter reading in electrical power distribution networks should be effected by mains-borne signals of the kind mentioned above. However, due to noise and other problems, the reliable detection and decoding of the digital signal in the mains-borne control signal is again extremely difficult.
It is therefore an object of the present invention to provide a receiver for receiving a carrier signal modulated with a digital signal, in which the abovementioned difficulty in detecting and decoding the digital signal is alleviated.